Emplacement mechanism and origin of the Yakuno Ophiolite

Yakuno蛇绿岩和其上覆的Maizuru群构成了日本二叠纪弧外的俯冲棱柱。支持这一概念的有：位于雅库诺蛇绿岩玄武岩上的远洋硅质岩，含硅凝灰岩的远洋泥岩，来自以前海山的外来石灰岩化石，以陆源砂岩和砾岩为终点的粗化向上层序，以及弧前盆地晚二叠世和三叠纪的轻度变形建造。然而，吸积棱柱最重要的方面是向海的叠置和重复，涉及Maizuru群和Yakuno蛇绿岩。D_1变形构造包括轴-平面面理(麦祖鲁群的压溶解理和雅库诺蛇绿岩的麻粒岩和角闪岩的变质层理)、扁平型应变、对称的压力阴影和边缘、具有轴向-平面面理的等斜褶皱，以及该褶皱的翻转翼部的逆冲断层。…更异常的不对称结构表明逆冲附近有向海方向的剪切。麦祖鲁带的D_1构造以剥离作用解释，位于底部滑脱之上。晚侏罗世D2构造为扭折褶皱，包括一级不对称扭折褶皱，在其倾覆的肢体处有脆性逆冲。D2发生在M2逆退变质过程中。M1递进变质作用是雅库诺蛇绿岩在麻粒岩和角闪岩层理中形成变质矿物的中压类型，也是麦祖鲁群沿泥岩压溶解理形成伊利石的中压类型。变质程度受地层或蛇绿岩地层位置控制，属于洋底变质作用。而M1递进变质作用仅作用一次，与D1剥离同时进行。俯冲带通常处于高P/T状态，因此一度处于异常变质条件下。如果炎热而年轻的大洋板块向日本弧下靠近，然后俯冲到日本弧下，就有可能出现如此高的温度梯度。雅库诺蛇绿岩可能形成于沟-沟-脊三重结合部。较少

The Yakuno ophiolite and overlying Maizuru Group constitute an acceretionary prism off the Permian Japan arc. This concept is supported by : pelagic chert resting on the basalt of the Yakuno ophiolite, herripelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from former-accreted seamounts, a coarsening upward sequence ending with terrigenous sandstone and conglomerate, and the mildly deformed latest Permian and Triassic formations of fore-arc basins. The most important aspect of an accretionary prism is however, seaward imbrication and repetition, involving both the Maizuru Group and the Yakuno ophiolite. The D1 deformation structures include an axial-planar foliations (pressure-solution cleavage for the Maizuru Group and metamorphic layering of granulite and amphibolite for the Yakuno Ophiolite),flattering type strain, symmetric pressure shadow and fringe, isoclinal folds with the axial-planar foliations, and a thrust fault at the overturned limb of this fold. … More Exceptional asymmetric structure indicates seaward directed shear near the thrust. The D1 structure of the Maizuru zone is explained by off-scraping, above the basal decollement. The Later Jurassic D2 structure is a kink fold, including a first order asymmetric kink fold with a brittle thrust at its overturned limb. D2 occurs at the time of M2 retrograde metamorphism. M1 prograde metamorphism is the medium-pressure type for the Yakuno Ophiolite that forms metamorphic minerals in layering of granulite and amphibolite ; and also for the Maizuru Group to form illite along pressure solution cleavage of mudstone. The metamorphic grade is controlled by the stratigraphic or ophiolite-stratigraphic position, and such metamorphism is a kind of ocean-floor metamorphism. However, the M1 prograde metamorphism acted only once, progressing contemporaneously with D1 off-scraping. The subduction zone, usually under high P/T regime, was thus, at one time, under abnormal metamorphic conditions. Such a high thermal gradient is likely if hot and young oceanic plate was aproaching and then subducting beneath the Japan arc. The Yakuno Ophiolite was probably formed at the trench-trench-ridge triple junction. Less